Lumber dryer and drying method



June 4, 1968 H. w. REYNOLDS 3,386,183

LUMBER DRYER AND DRYING METHOD Original Filed Jan. 31, 1964 srm vg I I8 amen AIR AMBIENT 10 16M 55::

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HUGH W. REYNOLDS ATTORNEYS United States Patent 3,386,183 LUIVEEEER DRYER AND DRYING METHGD Hugh W. Reynolds, Longview, Wash, assignor to Weyer haeuser (Iompany, Tacoma, Wash, a corporation of Washington outinuation of application Ser. No. 341,684, Jan. 31,

1964. This application Oct. 3, 1966, Ser. No. 584,003

4 Claims. (Cl. 3448) ABSTRACT OF THE DISCLQSURE A process and apparatus suitable for drying lumber which utilizes an en losure for the lumber, a blower, heating means for heating the incoming ambient air, sensing means for measuring wet-bulb temperature in the incoming air, sensing means for measuring the dry-bulb temperature of the drying air, and control means for maintaining a constant wet-bulb depression by controlling the heating means without changing the ambient water content of the air. With this equipment it is possible to dry lumber utilizing a constant wet-bulb depression of the drying medium without changing the ambient water content.

This is a continuation application of Ser. No. 341,684, filed Jan. 31, 1964, now abandoned.

This invention relates to the air-drying of lumber and more particularly to the forced heated air-drying of lumber.

Generally, air-drying is carried out by allowing ambient air to course through stacks of lumber using prevailing winds or other convective factors as a driving force. When circulation rates are low or variable the air-drying rate is consequently reduced. The present invention avoids this variation in drying rates by using a forced circulation technique which provides for the circulation of air through the lumber stack at a predetermined rate which is substantially independent of wind direction and/or velocity.

However, the drying rate is also affected by the moisture content of the circulating air as the humidity of the air determines both the equilibrium moisture content of the wood in contact with the air and the capacity of the air to pick up and transport moisture from the wood. In areas where high humid conditions occur at certain times, even to the extent of the lumber becoming more moist rather than more dry. For example, in a study made in the Pacific Northwest Coast area it was discovered that airdrying of cedar actually progressed only during the daytime hours of July, August and September. During the rer mainder of the year, the drying proceeded very slowly and in some instances the lumber actually increased in moisture content.

As a consequence, it is the usual practice to air-dry such lumber for periods of from 8 to 10 months in order to reach an average moisture content in the range of to at which time further drying is accomplished in a kiln. This long period of air-drying prior to completing the drying in a kiln makes it necessary to maintain large inventories and does not permit a lumber mill to respond quickly to changes in the market not previously anticipated. This is particularly burdensome Where certain species of lumber have'somewhat short and seasonal marketing periods.

Furthermore, where there is considerable variation in the climate during the year, air-drying will not produce a uniformly dried lumber and this non-uniformity is carried over into the kiln drying with the result that much of the lumber is either overdried or not dried sufliciently, thus creating further problems in attempting to produce a uniform grade of lumber.

3,385,183 Patented June 4, 1968 'ice In the past many attempts have been made to overcome the problems connected with pre-drying of lumber by using simply constructed dryers and circulating air of a predetermined temperature through lumber stacked within the dryer. Most of these dryers comprise an enclosure having a roof, sidewalls and at least one end wall. Means wereprovided for circulating and sometimes recirculating air through a heater and then through the lumber stacks. Further means were provided to control the humidity in the air, either by dilution with outside air of low humidity or by adding steam and water spray. All of these prior methods, although accomplishing the desired results, also proved to be quite expensive requiring as they did additional heating capacity at the plant site such as steam, gas or oil heat and also requiring expensive control systems in order to achieve the proper humidity conditions.

It is therefore an object of the present invention to provide a forced circulation air-dryer for the drying of lumber which is less expensive and easier to control than those heretofore proposed.

It is a further object of the present invention to provide an inexpensive and easily controlled system for air-drying lumber at a predetermined rate.

The present invention makes use of an enclosure comprising a roof and end walls in which stacks of lumber may be placed. At one end of the enclosure is a radiator of heated coils through which atmospheric air from a blower enters the dryer enclosure. The side walls of the dryer are open to the atmosphere sufiiciently to provide for the exit of the air after passing through the lumber stacks. The spaces which extend between the open side walls of the dryer and the adjacent lumber stack and the spaces extending between adjacent lumber stacks at each end of the enclosure are bathed to avoid by-pass of the air without passing through the stacks of lumber.

Other objects and advantages of the invention will be apparent from the following description and from the drawings in which:

FIGURE 1 is a perspective view of the apparatus and installation embodying my invention; and

FIGURE 2 is a schematic diagram of the control circuit.

Generally stated, the present invention is adaptable for low temperature drying of lumber in those areas which have a climate predominately of low temperatures and high humidity conditions. It is further based on the discovery that although the drying rate is largely determined by the velocity, temperature, and humidity of the air, maintaining a constant air velocity permits appreciable changes in the temperature and humidity with only negligible changes in the heat capacity.

Assuming that the velocity of the air remains constant, it was observed that over quite a temperature range the wet bulb depression was a partial index of the drying ability of the air. In other Words within quite a large temperature range if the wet bulb depression remains constant, the amount of heat available for drying per unit of air volume will be substantially constant. For example, taking the air velocity as a 100 c.f.m. and correcting to volume changes, we find that at a dry bulb temperature of 40 F. with a wet bulb depression of 15 F. the Btu/lb. of dry air available as the dry bulb temperature is reduced to the wet bulb temperature is 28.2. At a temperature of F. and 15 F. wet bulb depression the B.t.u./lb. of dry air available as the dry bulb temperature is reduced to the wet bulb temperature is 26.9.

Therefore, by varying the dry bulb temperature from 46 to 90 F. and keeping the wet bulb depression con- For all practical purposes this small variation can be considered negligible in the drying of lumber.

Referring to FIGURE 1, there is illustrated generally a shed or an enclosure 1 having a roof 2 and end walls 3 and 5 sufficiently large to contain at least one pile or stack of lumber. The end Wall 5 of the enclosure has an opening to which is attached a tunnel-like enclosure 6. The opposite end of the tunnel-like enclosure contains a constant speed blower or fan 8 open to the atmosphere. Between the fan 8 and the opening in end wall 5 is situated heating coils 7. A control box 5* is also attached to end wall 5. The size of the opening in end wall 5 and consequently the size of the tunnel structure, can of course vary depending upon such requirements as the size of the lumber stacks being dried, the number of them contained within the enclosure and the velocity of the fan and the type of heating means being used. The sides of the enclosure are open to the atmosphere to permit the escape of the air after passing through the stacks of lumber. In operation, baflies 29, 23, 21 and 22 may be placed at the top, bottom, and the ends of the enclosure in order to insure that the air goes throughthe lumber stacks and does not by-pass them.

Referring to FIGURE 2 which is a schematic diagram of the control circuit system used in the dryer operation and assuming that at the start of the drying period the heating coils are cold and the ambient air temperature is 40 F. with a wet bulb of 37.5 F. and that for efficient drying a 15 F. wet bulb depression is required, the control system would operate in the following manner. The dry bulb transmitter 10 sends a signal to the relay 11 proportional to the 40 F. temperature registered by the dry bulb thermometer 12. A manual set bias signal 13 proportional to a temperature of F. is subtracted from the dry bulb transmitter signal in the relay 11. The output signal of relay 11 thus becomes proportional to a temperature of F. and is transmitted to a controller 14 at the same time the wet bulb transmitter 15 sends a signal to the controller 14 proportional to a temperature of 37.5 F. registered by the wet bulb thermometer 16. The output signal of relay 11 is used as the set point and the signal from the wet bulb transmitter 15 as the input to the controller 14 which provides an output signal proportional to the difference between the set point signal and the input signal. This latter signal is transmitted to a control means 17 for heating or not heating the heating coils 18.

In the embodiment illustrated in FIGURE 2, steam is used to provide heat for the heating coils 13, and control means 17 is a steam valve. When the input signal from the wet bulb transmitter 15 is greater than the set point signal from relay 11, the output signal from the controller 14 acts to open the steam valve 17. Opening of the steam valve permits steam to circulate through the heating coils 18 and to heat the air passing through from blower 19 thus raising both the dry bulb and Wet bulb temperature. However, the dry bulb temperature increases faster than the wet bulb temperature so that when a dry bulb temperature of 65 F. is reached the wet bulb temperature will be only 50 F. When this occurs the set point signal and the input signal to controller 14- will balance and the output signal will modulate the steam valve to maintain control.

If during the drying cycle the ambient dry bulb temperature should fall for example, 5 F., the dry bulb transmitter signal will become proportional to temperature of 60 F. and the controller set point signal will become proportional to 45 F. Under these conditions the wet bulb temperature will be 48 F. and the wet bulb transmitter signal will be proportional to that temperature which will now be 3 F. above the set point signal, thus causing the steam valve to open further. This in turn will add extra heat to the air passing through the heating coils and raise the dry bulb temperature to 6 5 F. again, and the wet bulb will necessarily raise back to 50 F. again, giving the required 15 F. wet bulb depression.

When the ambient air conditions are such that a 15 F. wet bulb depression exists the valve will be closed cutting oil? the flow of steam to the heating coils and they will remain cold. The method of operation is as follows:

Assuming that the ambient air is at a temperature of F. dry bulb with a 65 F. wet bulb temperature and that a 15 F. wet bulb depression is desirable for the drying cycle, the dry bulb transmitter 16 sends a signal to relay 11 proportional to 85 F. The manual set bias signal 13 sent to the controller 14 is proportional to a temperature of 70 F. Meanwhile the wet bulb transmitter 15 sends a signal to the controller 14 equal to 65 F. Since the set point of controller 14 is 70 F. and is above the input signal received from the dry bulb transmitter of 65 F. the output signal closes the steam valve.

With this control system, both the controller input and the controller set point automatically change. When the set point of the controller 14 is below the input signal received from wet bulb transmitter 15 the steam valve is opened. Opening the steam valve permits the heating coils to heat the air passing through it and both the controller input and controller set point are increased. However, the set point increases faster than the input 50 soon both match and the steam valve is controlled to keep the two matched.

The invention is susceptible of numerous modifications without departing from the spirit thereof. Thus the dimensions of the enclosure may be varied within wide limits and the number and disposition of the baflies described may be varied depending upon the type of lumber being dried. The type and number of blowers may also be changed as required. The control instruments and heating means may be modified or changed depending on the requirements of the installation. In the embodiment illustrated, filled system thermometers combined with pneumatic instruments performed very satisfactorily. However, other heating means and electronic control instruments can also be used successfully.

Having thus described my invention I claim:

1. In a dryer for drying green lumber an enclosure comprising: 7

a roof, end walls, and side walls open to the atmosphere,

blower means mounted to pass atmospheric air at a constant velocity into said enclosure and through stacks of lumber disposed therein and to exhaust it through said open side walls, heating means mounted to heat the atmospheric "air prior to passing through said stacks of lumber,

control circuit means including means to measure the dry and wet bulb temperatures of the air passed through said heating means,

means associated with said measurement means to transmit a signal proportional to said dry and wet bulb temperatures,

relay means to receive said transmitted dry bulb temperature signal,

bias means to subtract a signal proportioned to a predetermined Wet bulb depression from said dry bulb temperature signal,

controller means for receiving said wet bulb temperature signal and said biased dry bulb temperature signal and for transmitting a signal proportional to the difference of said signals,

control means for receiving said controller signal and responsive to control the heating of said heating means, thereby maintaining a minimum predetermined wet bulb depression of the air passing into and through said stacks of lumber.

2. A dryer for drying green lumber comprising:

an enclosure designed to contain stacks of lumber and open to the atmosphere at both sides,

means positioned to pass atmospheric air at a constant velocity and at its ambient water content into said enclosure through said stacks of lumber and exhaust it through said open sides,

heating means positioned to heat said atmospheric air Without altering its ambient Water content prior to passing through said stacks of lumber,

control circuit means including means to measure the dry and wet bulb temperatures of the air passed through said heating means, and

means responsive to said temperature measuring means to control the heating of said heating means to maintain a predetermined wet bulb depression of the air passed through said heating means.

3. The dryer of claim 2 wherein said enclosure additionally includes:

baffle means disposed adjacent to the stacks of lumber to prevent air bypass.

6 4. The process of drying lumber comprising: passing atmospheric air at a constant velocity into and through a stack of lumber disposed in an enclosure; sensing the dry and wet-bulb temperature of the air entering said lumber stack; comparing the differences between said temperatures and a predetermined desired wet-bulb depression; heating said air without alteration of the ambient water content of said air; and controlling said heating to reach and maintain said wet-bulb depression.

References Cited UNITED STATES PATENTS 1,804,107 5/1931 Mueller 34230 FREDERICK L. MATTESON, 111., Primary Examiner.

20 ROBERT A. DUA, Examiner.

D. A. TAMBURRO, Assistant Examiner. 

